In the industry of electrical-to-optical light emitting devices, such as, for example, lasers and light emitting diodes (LEDs), attempts are continuously being made to improve the efficiency with which electrical power input to the device is converted into light, which is known as quantum efficiency. In order to improve the external quantum efficiency, ηext, of a light emitting device such as a laser or an LED, it is desirable to maximize the injection efficiency, ηinj, into the active region and enhance the radiative recombination, ηrad, while maximizing the extraction efficiency ηout out of the device. The overall external quantum efficiency is defined by the equation ηext=ηinjηradηout. The term ηinj can be made close to unity by the introduction of hetero-barriers and multiquantum well active regions while the term ηrad can be made close to unity by reducing the sources of non-radiative centers in poor quality epitaxial materials. The extraction efficiency ηout for planar LEDs is limited by Snell's law to around 2 to 4% due to the large refractive index discontinuity at the semiconductor/air interface. Various devices and techniques, such as diffractive Bragg reflector (DBR) mirrors, textured surfaces, photon recycling, pyramidal shaping, and radial outcoupling tapering, are used to increase the extraction efficiency. Some of these devices and techniques have resulted in better than 50% improvement in extraction efficiency.
The main disadvantage of these devices and techniques is that they only enable a small fraction of the optical modes to be extracted from the light emitting devices. Leaky modes and guided modes are lost due to reabsorption of electrons in the unpumped active region. The number of optical modes can be controlled by reducing the size of the optical volume to the dimension of a wavelength. Introduction of a photonic crystal structure, also known as a bandgap (PBG) structure, into a light emitting device has been proposed to inhibit both the guided and leaky modes. A photonic bandgap structure controls the number of optical modes by introducing periodic reflections for all propagation directions. The periodic variations in the index profile generate a bandgap in the frequency domain where no optical modes are allowed. If there are no available optical modes, there can be no spontaneous emission. The PBG structure is designed to have the forbidden band at the transition frequency of the spontaneous emission. By introducing a defect (i.e., a missing period) in the photonic bandgap structure, the number of allowed optical modes can be controlled. Depending on its dimensions, a single defect in the photonic bandgap structure can be designed that allows a single optical mode to exist and that ensures that all of the spontaneous emission generated by electron-hole recombination can couple only to this mode. If this mode couples light efficiently out of the light emitting device, it will have very high external quantum efficiency.
LEDs with two-dimensional (2-D) PBG structures have been created. The 2-D PBG is typically formed by etching holes into the LED emission surface. For example, in an article entitled “InGaN/GaN QW heterostructure LEDs employing photonic crystal structures” published in Applied Physics Letters 84, 3885 (2004), J. Wierer et al. describe such a device. These methods have not resulted in tremendous enhancements in the external quantum efficiency of LEDs, primarily because of the way in which the PBG is fabricated. In all cases, etched holes are use to generate the periodic index variations, which result in 1) poor injection efficiency into the active region, and 2) increased non-radiative recombination at the etched surface (i.e., surface states). In addition, bottom reflectors are used to extract the light output, which is not extremely efficient mechanism for extracting light.
Accordingly, a need exists for a photonic crystal structure that has improved external extraction efficiency and that does not rely on creating a 2-D PBG by etching holes into an LED emission surface.